The present invention relates to a wafer stepper for fabricating semiconductor ICs (Integrated Circuits) and, more particularly, to a wafer stepper for forming a fine pattern on a semiconductor substrate accurately.
In the semiconductor ICs art, the degree of integration is quadrupling every three years, thanks to fine circuit pattern technologies. For example, to form a fine VLSI (Very Large Scale Integrated circuit) pattern on a semiconductor substrate, use is made of reduced-scale projection exposure capable of forming the fine pattern on the substrate accurately. A device for effecting the reduced-scale projection exposure is usually referred to as a wafer stepper, or simply stepper, and basically comprised of a mercury lamp or similar light source, reflector, first convex lens, fly's eye lens, aperture, second convex lens, a mask having a fine pattern, and a projection lens.
In the wafer stepper, light issuing from the light source is condensed by the reflector, uniformalized by the first convex lens and fly's eye lens, restricted by the aperture to a suitable size, and then incident to the mask via the second convex lens. The light incident to the mask is diffracted by a fine pattern, i.e., chromium pattern. As a result, zero order light, +first order light and -first order light are incident to the projection lens. Diffracted lights of second order and above advance in a direction angled more than the zero order diffracted light and are, therefore, usually not incident to the projection lens. The diffracted lights are focused onto a wafer or substrate and forms an aerial image identical with the chromium pattern in a reduced scale.
To meet the increasing demand for fine IC patterns, the stepper should be provided with an ability to expose finer patterns, i.e., resolution. The resolution R of a stepper is expressed by using the Rayleigh's equation, as follows: EQU R=K.sub.1 .lambda./NA
where .lambda. is the wavelength of exposing light, NA is the aperture number of the projection lens, and K1 is a constant determined by resist performance. The above equation indicates that for higher resolution, it is necessary that the wavelength of the exposing light be reduced, or that the aperture number NA of the projection lens be increased. When the wavelength .lambda. is reduced, only excimer laser beams, including KrF excimer laser beam (249 nm) or ArF excimer laser beam (193 nm), are available-which have wavelengths shorter than that of the conventional g beam (463 nm) and i beam of a mercury lamp and have illumination necessary for exposure. However, the problem with an excimer laser is that it requires gas and parts to be replaced often. Moreover, this kind of laser is more than hundred times as expensive as a mercury lamp, increasing the cost of semiconductor ICs. Shorter wavelengths would make it difficult to design and fabricate projection lenses, among others, since optical parts, optical materials and resists having high transmittance are rare. On the other hand, the aperture number NA of the projection lens cannot be increased beyond a certain limit since increasing it results in a decrease in the depth of focus D. The depth of focus D is also determined by the Rayleigh's equation, as follows: EQU D=K.sub.2 .lambda./(NA).sup.2
where K2 is a constant determined by a resist. It will be seen that the depth of focus D decreases in inverse proportion to the square of NA. Hence, NA cannot be increased beyond a certain limit.
To eliminate the above problems, two different exposing methods have been proposed. Specifically, Japanese Patent Laid-Open Publication No. 4-343215 discloses a method using a grating or similar optical element having a periodic structure or pattern located just above the mask, so that exposing light may be incident obliquely to the mask. In a device for implementing this oblique illumination, light incident to the optical element is diffracted by the periodic pattern of the optical element. The periodic pattern is so configured as to cancel zero order components. As a result, the +first order components and -first order components are incident to the mask. While three beams, i.e., zero order beam, +first order beam and -first order beam have customarily been focused onto the wafer due to the absence of the optical element, this stepper focuses two beams, i.e., +first order beam and -first order beam onto the wafer and, therefore, achieves a high resolution without reducing the depth of focus. Specifically, even if the aperture number NA of the projection lens is small, the resulting image appears as if it were exposed by a projection lens having a great aperture number. However, the stepper is operable only with the optical element having a checkerboard pattern, i.e., an arrangement of squares. The pattern is ineffective for 45.degree. tilted features often used with semiconductor ICs, and moreover it lowers resolution and the depth of focus, obstructing the fabrication of ICs.
The other exposing method is taught in Japanese Patent Laid-Open Publication No. 4-369208. This method uses an aperture having an opening in: a peripheral portion thereof apart from the center. In this configuration, beams passed through the opening are incident obliquely to the mask. The opening of the aperture has an area which is less than one half of the area of a usual opening. As a result, the aperture needs an exposure time twice as long as that of the ordinary aperture in exposing the wafer. This makes it difficult to fabricate ICs at low cost.